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Are your safeguards as safe as you think?

09 July 2012

There are doubtless many machines in the UK fitted with multiple guards that are monitored in one circuit by series connected safety switches with dual channel wiring; does this sound like one of your machines? Can any of these guards be opened simultaneously? Then read on, advises David Collier.

Historically the practice of series-wired safety switches has arisen because it saved money on cabling and safety relays, and because such dual channel wiring translated to Category of 3 of the now-withdrawn standard EN 954-1 (for more than one switch in series, EN 954-1 degraded Category 4 to Category 3).

Category 3 lives on in the standard EN ISO 13849-1 in which clause 6.2.6 requires that for Category 3 to apply, specific conditions must be met, including: a single fault must not lead to a loss of the safety function; that an accumulation of undetected faults can lead to the loss of the safety function; and, importantly as an addition over and above EN 954-1’s requirements, that at least 60% of faults have to be detected in a diagnosis mechanism.

On closer inspection the ability of a system to detect 60% of dangerous faults can be impacted by a phenomenon known as ‘fault masking’, which can dramatically reduce the Diagnostic Coverage (DC) and consequently the Performance Level as will be explained below.

Fault masking
The answer as to how many (if any) switches can be connected in series depends on the faults that can be anticipated (of which there is a list in the validation standard EN 13849-2). The following example of interlocked guards connected in series is intended to illustrate this point (see Figure 1).

[1 ] three safety gates are connected in series to an evaluation device. Initially all the safety gates are closed and the relay’s outputs are ‘on’; ie, the machine can be operated. [2] On the left-hand safety gate, a short circuit occurs in the line to the switch with the N/C contact. At first, the fault is not detected (because a demand has not yet been placed upon the safety function) and the machine can continue operating (because the guard is still closed).

[3] The left-hand safety gate is then opened, an event which the left switch signals to the relay. During feasibility comparison of the two switches the safety relay discovers an inconsistency and switches to a fault condition; ie once the safety gate is closed the machine cannot be restarted (but in this case the safety gate is left open). [4] Now the right-hand safety gate is also opened. Via these signals the relay once again detects a normal condition. The fault condition is reset, the safety gates can once again be closed from left to right and the machine is ready to start up again.

This example illustrates an undetected fault in the safety circuit, which has built up as a result of the clearing of the fault by the simultaneous opening of two gates. An additional, subsequent fault could cause the whole interlocked guard system to fail to danger (eg, another wiring fault occurs, a guard is opened and the machine does not stop).

While this is in line with Category 3 (an accumulation of undetected faults can lead to a loss of the safety function) these and similar faults are described by the term ‘fault masking’. In the current standard EN ISO 13849-1, the maximum DC that the switch can achieve is restricted, depending on the masking probability.

In practice, a single switch pair that is evaluated by a safety relay can achieve a DC of 99%. Based on this premise, in the current draft of EN ISO 14119, the maximum DC for a group of interlinked switches is dependent upon the number of switches connected in series and their frequency of operation. Note at some point ISO 14119 will replace the current standard for interlocking, EN 1088. As can be seen in Table 1, masking restricts the maximum achievable DC and PL.

From the above, if it can be shown that no two guards are moved with a frequency of greater than once an hour, or there are no more than four of them in series, the statistical chance of a fault occurring and being masked is reduced; however, the DC of the system is reduced from 99% to 60% (low), which in terms of EN ISO 13849-1 means the best PL achievable is PL d, which also means Category 3 has been met. So there is no problem if the risk assessment required Category 3.

If it is found that more than one guard can be moved with a frequency of greater than once an hour, or there are more than four of them in series, the statistical chance of a fault occurring and being masked is high and the result is that DC is reduced to less than 60% (according to EN ISO 13849-1 this is equivalent to no DC). Under these circumstances, according to EN ISO 13849-1, the best achievable PL is PL c, or Category 1 in old terms. If the original risk assessment required Category 3, under these circumstances the system is no longer compliant.

Is there a cure for fault masking?
If a series of inter-linked switches is required to meet PL e, a technical solution is required, using switches with integrated fault detection. As masking cannot occur in this case, it is possible to have interlinked switches without restricting the DC or PL. Only switches with internal diagnostics and an OSSD (Output Signal Switching Device) output, a solid state type as commonly found on RFID based switches, are unaffected by this.

Such devices are certified by the manufacturer with PL e (ie, they are classed as a subsystem, not just a component) which means they have their own internal dual channel category 4 architecture, built in 99% DC, as well as the other internal characteristics allowing the series connection of switches (such as extremely low failure rates expressed as PFHD in the magnitude of 10-9 dangerous failures per hour).

Diagnostics of which guard has been opened (not to be confused with Diagnostic Coverage, which is to do purely with detection of dangerous failures) is provided on the switch body by LED status, and also via signalling which can be taken to a standard PLC.

Devices with RFID coding and OSSD outputs
Some manufacturers of safety components deploy this technology in their products. Other than the capability to avoid fault masking, RFID based non-contact switches also offer less troublesome switching (when compared with magnetic types) through various actuator approach angles, and better resistance to defeat through the use of varying degrees of coding (all the way to unique actuator/receive pairs), and better protection against ingress (when compared with mechanically actuated switches).

The use of distributed I/O
Other than replacing designs using series, volt-free switches with RFID/OSSD-based technology, there are other options based upon improved wiring management through ‘zoning’. Normal volt-free contact-based switches are wired individually, but in low numbers, back to local IP20 I/O modules in small control boxes (Pilz PDP20 F mag is an example), which in turn can be cascaded across the machine back to a main panel using the OSSD outputs of such modules to provide 99% DC throughout the system.

Where the luxury of enclosures for IP20 I/O modules is non-existent, I/O modules can be conveniently placed directly on-machine - such as Pilz PDP67 F 4 Code (see Figure 2) and PDP67 F 8DI ION (see Figure 3) because they are IP67 rated. These modules can be cascaded across a machine on one multicore cable back to the main control panel without degradation of DC or PL through use of coding or test pulses.

E-stops in series? 
It is worthy of note that series connection of Emergency stop devices is unlikely to incur a loss of DC, based upon the fair assumption that it’s unlikely that any two E-stops will be actuated simultaneously or as frequently as once an hour. Therefore it is reasonable to wire such devices in series. That said, it is generally inadvisable to require E-stops to perform to PL e simply because they’re not intended as primary protective devices; if a hazard requires a safety related control function to perform to PL e other primary means of safeguarding should be used.

Fault masking is a real issue even if you don’t refer to current or future standards and you just apply basic engineering logic. Designers of safety guards and associated circuits on new machines, and those responsible for existent machines in use should review whatever safety guard circuits they have where safety switches are connected in series. You need to ensure that the ugly head of masked faults can’t, sometime in the future, rise up and bite unsuspecting victims. The technology is available to help reduce on-machine cabling and, critically, the possibility of fault masking.

David Collier is with Pilz Automation Technology

PLEASE NOTE: For diagram/table, please refer to digital issue

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